are you using a headset with a microphone or just the computers built in microphone and speakers?
Ok, that is most likely your issue. There are two options you can try, the best option would be to purchase a headset mic for your pc. any usb headset will work , something like this for example http://www.amazon.com/Cyber-Acoustics-Neckband-Microphone-AC-634/dp/B0002VO7HO/ref=sr_1_8?s=electronics&ie=UTF8&qid=1336320249&sr=1-8
or you can first try using headphones (same you would use with an ipod or other media device) and the built in computer microphone. that might help as well.
What is happening is the pc is trying to block out the echo and cannot always tell your voice from background noise so it cuts out.
I've always known I could get a headset and mike, what I'd pay the $38 for is to fix the problem with my computer. Since you cannot repair it I would like a refund to my Mastercard!
That's perfectly fine. I will ask for a refund on your behalf. The answer to your question was given though, this is the case when using the built in mic and speakers.